1. Effects of burning on the photosynthetic rates of Camassia quamash
Claire Cook*, Nikko Bowen, Savannah Richard, Josh Spiegelman
The Evergreen State College
Abstract
Periodic fires influence the productivity and nutrient cycling processes of
prairie ecosystems. Increased productivity following infrequent fire is due to release
from multiple resource constraints, namely light and nitrogen. The low water and
nitrogen requirements characteristic of C4 grasses suggest burning would have less
of an impact on grasses than forbs, whose access to light and nutrients is largely
determined by the productivity of the dominant grasses. The purpose of this study
was to examine the photosynthetic rates of Camassia quamash, a perennial
herbaceous forb native to the prairies of the south Puget Lowlands (Washington,
USA), under varying burn regimes and to assess the possible contribution of soil and
foliar nitrogen content to the observed photosynthetic rates. Productivity of C.
quamash progressively declined with increasing number of years since burning.
Although soil and foliar nitrogen levels did not vary significantly with number of
years since burning, the high photosynthetic rates of the 2013 burn year may be
attributable to the high percentage of nitrogen in the soil and foliage.

2. Effects of burning on the photosynthetic rates of Camassia quamash
Claire Cook*, Nikko Bowen, Savannah Richard, Josh Spiegelman
Introduction
Periodic fires influence the productivity and nutrient cycling processes of
prairie ecosystems (Blair 1997; Knapp 1985; Ojima et al. 1993; Turner et al. 1997).
Shifts in resource limitation following intermittent fires can temporarily increase
aboveground productivity in prairie ecosystems, with the greatest rates of
productivity occurring in the growing season following a spring fire in an
infrequently burned prairie (Blair 1997; Briggs et al. 1994; Seastedt et al. 1991).
Increased productivity following infrequent fire is due to release from multiple
resource constraints, namely light and nitrogen (Blair 1997; Seastedt et al. 1991).
Intermittent fires induce a shift from a system limited primarily by energy due to
shading effects from accumulated detritus, to one limited by nitrogen availability
due to higher inputs of low quality plant residue (Blair 1997; Turner et al. 1997).
Volatilization of nitrogen during burning is a major source of nitrogen loss in prairie
ecosystems (Ojima et al. 1994), thus soil nitrogen availability was found to be
greatest on unburned sites and lowest on annually burned sites (Blair 1997).
Infrequent fires result in an increase in nitrogen availability with the onset of
nitrogen limitation occurring over time, causing nitrogen availability that is
intermediate between annually burned and unburned sites (Blair 1997; Ojima et al.
1994).

3. The low water and nitrogen requirements characteristic of C4 grasses suggest
that burning would have less of an impact on grasses than forbs; whose access to
light and nutrients is largely determined by the productivity of the dominant
grasses (Turner and Knapp 1996). Most prairie studies have focused on the effects
of burning and drought on net photosynthetic rates in both grasses and forbs (Blair
1997; Heckathorn et al 1997; Knapp 1985; Knapp and Gilliam 1985; Ojima 1994;
Seastedt et al. 1991; Turner et al. 1997; Turner and Knapp 1996), but few have
examined the effects of burning on the photosynthetic rates in an individual forb.
Historically, infrequent fires were instrumental in maintaining the structure and
function of prairies in the south Puget Lowlands (Washington, USA). It is thought
that indigenous peoples cultivated these prairies through the use of intermittent
fires, set at 2-3 year intervals (Storm and Shebitz 2006). Fire suppression following
Euro-American settlement resulted in a drastic reduction of native prairie
ecosystems, as well as invasive species encroachment on remaining prairies (Storm
and Shebitz 2006). Controlled burning has been reintroduced in an attempt to
restore native biodiversity, yet the effects of fire on the productivity of prairie plants
in the Pacific Northwest is not well understood.
The purpose of this study was to examine the photosynthetic rates of Camassia
quamash under varying burn regimes and to assess the possible contribution of soil
and foliar nitrogen content to the observed photosynthetic rates. We hypothesize
that recently burned plots will have higher photosynthetic rates as well as higher
soil and foliar nitrogen levels.

4. Methods
Glacial Heritage Preserve is a 1000-hectare area of native south Puget Lowland
prairie (46.871656° Latitude, -123.040493° Longitude). The Nature Conservancy
manages the study area, with portions of the prairie undergoing prescribed burning
every year. The plant community consists of many native grasses and forbs,
including Festuca idahoensis ssp. roemeri and Camassia quamash (Rook et al. 2011).
The site receives an average annual rainfall of 146 cm and has an average annual
temperature of 10.7° C (Western Regional Climate Center 2010).
C. quamash (Liliaceae) is a perennial herbaceous forb native to the prairies of the
south Puget Lowlands (Washington, USA). It grows from a deep-seated bulb, which
was traditionally cultivated as a wild edible (Hitchcock and Cronquist 1973). It is in
flower from May to June, and has largely senesced by early July.
Research was conducted at the Glacial Heritage Preserve in early May 2014. The
early spring study date allowed for analysis of productivity during a period when
water was not limiting and C. quamash was undergoing peak aboveground biomass
accumulation in order to complete its life cycle. Treatment sites were selected from
different burn years: 2011, 2012, 2013, and an unburned control plot. All of the
treatment sites had remained unburned for two to five years.
Net photosynthesis, transpiration, and stomatal conductance of fully expanded C.
quamash leaves were measured using an LC Pro+ Infrared Gas Analyzer (ADC
BioScientific Ltd. Hoddensdon, UK). The Infrared Gas Analyzer was programmed to
deliver a sequence of photosynthetically active radiation ranging from 80 to 870
μmol m-2 s-1. Three measurements at 870 PAR were collected for each C. quamash

5. individual. These values were averaged to generate an accurate measure of
photosynthetic rate at maximum PAR.
In order to determine the soil and foliar nitrogen content, a subsample of soil
cores and C. quamash leaves were collected at each treatment site. Soil and leaf
samples were oven dried at 50 C. Soil samples were sieved to pass through a 20 μm
mesh screen. Soil and foliage samples were analyzed for carbon and nitrogen
content using a 2400 CHNS Elemental Analyzer (Perkin-Elmer, Waltham, USA).
Analysis of variance (ANOVA) was utilized to compare the effects of burning
regime on net photosynthesis, transpiration, stomatal conductance, and water-use
efficiency using JMP Pro 10 software (SAS Institute 2014). The level of significance
for all tests was P < 0.05.
Results
Comparisons of the photosynthetic rates to incident photosynthetically
active radiation indicated
the photosynthetic rate of
C. quamash was greatest
in the 2013 burn plot and
least in the unburned
control plot (Fig. 1). These
differences are assumed
to result from the fire
treatment since the plots
Fig. 1. Response of photosynthetic rate to incident PAR for leaves of
Camassia quamash in May 2014. Leaf temperatures during
measurements were 18.5-30.4°C. Each error bar is constructed using 1
standard error from the mean.

6. were similar to one another prior to burning. Average photosynthetic rates at 870
PAR were significantly greater in the 2013 burn plot followed by the 2012 and 2011
burn plot, with the unburned control having the lowest photosynthetic rate (Fig. 2).
Average stomatal conductance at 870 PAR
was greatest in the 2013 burn plot, and least
in the unburned control plot (Fig. 3). Water
use-efficiency at 870 PAR was greatest in the
2013 burn plot and least in the 2011 burn
plot (Fig. 4). The high water use-efficiency in
the unburned control plot can be attributed
to the low transpiration rates of C. quamash
in the unburned plot. The transpiration rates
at 870 PAR did not show a significant
relationship across burn regimes. This is
likely due to the variation in leaf
temperatures at the time of measurement.
Minimum leaf temperatures were recorded
in the 2013 burn plot and maximums were
recorded in the 2011 burn plot.
Soil and foliar percent nitrogen was
not significantly different across burn
regimes. The highest soil and foliar percent
nitrogen values were recorded from the 2013
Fig. 2. ANOVA of average photosynthetic rate
by burn regime. Net photosynthesis values
were obtained under PAR levels representative
of full sunlight.
Fig. 3. ANOVA of average stomatal conductance
by burn regime. Stomatal conductance values
obtained at PAR levels representative of full
sunlight.
Fig. 4. ANOVA of average water use efficiency
by burn regime. Water use-efficiency values
obtained at PAR levels representative of full
sunlight.

7. burn plot. Second greatest foliar percent nitrogen values were found from
individuals growing on the unburned control plot, with the lowest foliar percent
nitrogen values
recorded at the
2011 burn plot.
Second greatest
soil percent
nitrogen values
were recorded on the 2011 burn plot, with the lowest found on the 2012 burn plot
(Table 1). The highest foliar C:N ratios were found on individuals growing in the
2011 burn plot with the lowest occurring in the 2013 burn plot. Low C:N ratios are
representative of high nutrient quality leaf material, which are typically found in
nutrient-rich environments.
Although soil and foliar nitrogen content did not differ significantly across
burn regimes,
treatment sites with
the highest
photosynthetic
rates also had the
greatest soil and
foliar nitrogen
content (Fig. 5).
Table 1. Average photosynthetic rate and soil and foliar nitrogen content across
burn regimes.
Fig. 5. Average photosynthetic rate as a function of foliar nitrogen content

8. Discussion
Number of years since burning significantly affects the productivity of C.
quamash. In this study, C. quamash individuals from a plot burned the previous year
exhibited the greatest rates of photosynthesis, stomatal conductance, and water use-
efficiency. These metrics of productivity progressively declined with increasing
number of years since burning. Our results are consistent with those of Seastedt et
al. (1991), Briggs et al. (1994), and Blair (1997) who found the greatest rates of
aboveground net primary productivity during the growing season following a spring
fire in an infrequently burned prairie. The low productivity metrics recorded for our
unburned plot may be reflective of energy limitations characteristic of unburned
prairies.
Seastedt et al. (1991) suggest that the increase in productivity can be
attributed to increased nitrogen availability. This is reflected in the high soil and
foliar percent nitrogen recorded at the 2013 burn plot. Blair (1997) found that
nitrogen availability declines following an infrequent burn, resulting in nitrogen
availability that is intermediate to unburned and annually burned plots. Our data
suggest that nitrogen availability is progressively declining with time since burning.
It appears that the 2012 and 2011 burn plots are approaching intermediate nitrogen
availability levels due to the fact that they have foliar percent nitrogen levels that
are less than the unburned plot.
When comparing the productivity of the unburned control plot and the 2012
and 2011 burn plots, one would expect the productivity values to be in-line with soil
and foliar percent nitrogen levels. The high productivity of the 2012 and 2011 burn

9. plots in spite of lower soil nitrogen availability, in the case of the 2012 burn plot,
and lower foliar percent nitrogen, in the case of both treatment plots, may result
from a combination of increased nitrogen use-efficiency and shifts in carbon
allocation patterns (Ojima 1994).
Most studies concerned with the response of prairie ecosystems to fire have
been at the community level (Blair 1997; Heckathorn et al 1997; Knapp 1985;
Knapp and Gillman 1985; Ojima 1994; Seastedt et al. 1991; Turner et al. 1997;
Turner and Knapp 1996). Since Camassia quamash is one of the dominant forbs at
Glacial Heritage Preserve, its productivity and physiological response to fire should,
to an extent, be representative of the other dominant and subdominant forbs in this
ecosystem.
An important consequence of infrequent fire in prairie ecosystems is the
removal from multiple resource constraints. In the absence of fire, prairie
ecosystems are largely energy limited, thus inorganic and mineralizable nitrogen is
allowed to accumulate in the form of leaf residue and detritus. Following fire, an
adequate energy environment enables these nutrients to be utilized (Ojima et al.
1994; Towne and Owensby 1984). Over time, the soil nitrogen availability will
decline to levels intermediate to those of unburned and annually burned plots. This
response is in line with those of a system in transition from a state of energy
limitation to a state of nitrogen limitation (Blair 1997).
Intermittent fires were historically influential in shaping the structure and
function of south Puget Lowland prairies. In addition, these fires have been shown
to impact the productivity and nutrient cycling in these ecosystems, with greater

10. early-season production occurring in the growing season following a burning.
Increased uptake of soil nitrogen prevents leaching losses from the system and
enables the more efficient use of this nitrogen in acquiring carbon (Knapp 1985;
Hayes 1985). Infrequent fires are a valuable management practice that potentially
reduces post-burn nitrogen losses from the system, as well as encouraging the high
productivity that is characteristic of these threatened ecosystems.
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